Re: Possible nohz-full/RCU issue in arm64 KVM

Hi Mark,

On Tue, 2022-01-04 at 16:39 +0000, Mark Rutland wrote:

On Fri, Dec 17, 2021 at 04:54:22PM +0100, Paolo Bonzini wrote:

quoted

On 12/17/21 15:38, Mark Rutland wrote:

quoted

For example kvm_guest_enter_irqoff() calls guest_enter_irq_off() which calls
vtime_account_guest_enter(), but kvm_guest_exit_irqoff() doesn't call
guest_exit_irq_off() and the call to vtime_account_guest_exit() is open-coded
elsewhere. Also, guest_enter_irq_off() conditionally calls
rcu_virt_note_context_switch(), but I can't immediately spot anything on the
exit side that corresponded with that, which looks suspicious.

rcu_note_context_switch() is a point-in-time notification; it's not strictly
necessary, but it may improve performance a bit by avoiding unnecessary IPIs
from the RCU subsystem.

There's no benefit from doing it when you're back from the guest, because at
that point the CPU is just running normal kernel code.

I see.

My main issue here was just that it's really difficult to see how the
entry/exit logic is balanced, and I reckon we can solve that by splitting
guest_{enter,exit}_irqoff() into helper functions to handle the vtime
accounting separately from the context tracking, so that arch code can do
something like:

  guest_timing_enter_irqoff();
  
  guest_eqs_enter_irqoff();
  < actually run vCPU here >
  guest_eqs_exit_irqoff();
  
  < handle pending IRQs here >
  
  guest_timing_exit_irqoff();

... which I hope should work for RISC-V too.

I've had a go, and I've pushed out a WIP to:

  https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/kvm/rcu

Had a look at the patches and they seeem OK to me.

Thanks!

-- 
Nicolás Sáenz


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